Springtime, for most of us in northern temperate climes, is a welcome season of blooming plants, longer days, and warmer temperatures. But for Ted Schuur, an ecologist at the University of Florida, the season has different connotations. For Schuur, spring means a new season of exploration and adventure on the geographic and scientific frontier, where he is trying to answer a question of immense importance for our warming planet: How much carbon is being released by the thawing of the Arctic’s so-called permafrost?

Permafrost is the frozen soil, water, and rock that, astonishingly, covers nearly one quarter of all the exposed land surface in the Northern Hemisphere. Trapped within this frozen ground is a vast amount of carbon, which has accumulated from the partial decomposition of plants and animals over tens of thousands of years. Frozen in permafrost, the organic matter is mostly stable, decaying very, very slowly. But as air and permafrost temperatures rise, the organic material breaks down more rapidly and releases carbon into the atmosphere at a higher rate. Schuur’s research seeks to find out precisely how much carbon is released in this process and how rapidly the thawing is occurring.

The stakes of this research could hardly be higher. Schuur and his colleagues have estimated that the northern permafrost contains nearly 1.7 trillion tons of carbon. That’s roughly twice the total amount of carbon in the atmosphere today. If even 20 percent of the northern permafrost thaws, Schuur explains, it could release as much carbon into the atmosphere as humans have emitted by burning fossil fuels since the beginning of the Industrial Revolution.

Just how much carbon will the permafrost release? To understand the process more precisely, each year around this time, as soon as school ends, Schuur and his family pack up their home and make the trek to a rustic cabin in Alaska with no running water. Schuur has spent each of the past 14 summers closely monitoring the region’s permafrost. “In some sense,” he says, “my research allows me to live out my childhood dream of camping out as part of my job.”

Growing up in Michigan, Schuur says, he was always interested in nature. He fondly recalls his years as a Boy Scout, and he loved going to “insect camp,” where he was encouraged to catch bugs and learn about biology. Later, “in my graduate work,” he explains, “I focused on tropical rainforests, where I became fascinated by the ecological dynamics of the carbon cycle” — the way carbon moves between the soil, living plants, animals, and all forms of life as well as the ocean and the atmosphere. Ultimately, Schuur’s fascination with the carbon cycle led him to study the Arctic. “It was a case of applied science,” he says. “The simple fact is, exchanges of carbon in Earth’s northernmost regions have a potential for particularly large effects on the planet’s climate.”

In one ongoing study of thawing permafrost, Schuur’s research team makes detailed measurements at dozens of small, open-topped research chambers set up in the Alaskan wilderness. Each chamber is less than two feet tall — but large enough to enclose the typically small tundra vegetation. By lowering the station’s moveable roof for two minutes at a time following a precise protocol, Schuur measures the amount of carbon that is taken up and released at each location at all times of the day and night throughout the spring and summer and fall.

Schuur’s latest efforts takes his past observational research one step further. His research team has begun to artificially warm some of the plots, both to test his past results and to more precisely predict what to expect as further warming occurs.

In Schuur’s research, two key questions have important implications for global warming trends. The first is: To what extent does the release of carbon follow a linear trajectory as permafrost warms? The top layer of permafrost is “active,” in that it thaws and freezes seasonally and is naturally more susceptible to changes in surface temperature. Schuur is trying to understand whether certain temperatures or conditions at the surface might trigger disproportionately greater releases of carbon from the layers below.

He also studies the form the carbon takes when it is released, a very important factor. Organic matter that degrades with plenty of air supply emits primarily carbon dioxide. But when soil is very wet or under water where there is little available oxygen, decomposing organic materials give off methane gas in combination with carbon dioxide. The composition of the gases released makes a huge difference, because methane, though much less abundant in the atmosphere than carbon dioxide, traps some 25 times more heat than carbon dioxide when compared over a century timescale. Carbon released as methane lasts in the atmosphere for about a decade before converting to carbon dioxide, which continues to trap heat for many more decades, some of it lingering for many centuries. “The distinction [between the two gases] gives us some very important information about the impact of permafrost carbon on the pace of climate change,” Schuur says, “but the fact is, there is going to be a significant impact whether carbon is released as carbon dioxide or as methane.”

Schuur’s research takes on added urgency because global warming is occurring twice as fast in the Arctic as most places on Earth. In Alaska you don’t have to be a scientist like Schuur to see the effects firsthand. Average temperatures in Alaska have risen by 3 to 5 degrees Fahrenheit over the past 30 years.

As greenhouse gases — either carbon dioxide or methane gas — are released into the atmosphere from permafrost, scientists predict a vicious cycle: The gases will trap heat and warm the air, thawing more permafrost, releasing more carbon and further heating the planet. Scientists call this a “positive feedback” because it speeds up the pace of change. But for the current pattern of life on Earth, there is nothing positive about it.

Schuur’s work is crucial because it helps us understand more precisely the impact of thawing permafrost on the pace of climate change. It is important to note that while the exact extent and speed of the changes to come are not known, there is no uncertainty about the general warming trend. As a ballpark figure, Schuur says “it is reasonable to assume that 100 billion tons of carbon and perhaps more could be released into the atmosphere from thawing permafrost this century.” That amount, equal to about a third of the total carbon emissions humans have caused so far, is cause for substantial concern.

But, despite the scale of the looming thaw, Schuur says he sleeps well at night.

“The way I see it,” he says, “thawing permafrost carbon is a huge unknown process already underway, and my mission is to tell the world everything I can about it. My goal is not to scare people but to allow us all to look into the future with as much information as possible. As immense as the consequences may be, I feel my work has a positive purpose. I do believe that the more we know, the more control we have over our future.”

This is the 17th installment of America’s Climate Scientists: A series from the Union of Concerned Scientists. Click here to read all the climate scientist profiles.

The Union of Concerned Scientists is currently leading a campaign to elevate the voices of climate scientists and educate the public about the overwhelming scientific evidence for human-caused global warming. Learn how you can get involved at www.ucsusa.org/evidence
.

Most people are understandably confused about the relationship between global warming and natural variability in the weather. After the huge snowfalls in the northeastern United States over the past few months, for instance, many people can’t help but wonder: With a winter of such magnitude, how can scientists say the planet is warming?

Day-to-day and seasonal weather fluctuations present challenges not only for the public but also for climate scientists trying to tease apart the relationship between long-term climate change and weather variability. Wenhong Li, an atmospheric scientist at Duke University, studies precisely this link. Her latest research on precipitation patterns in the southeastern United States offers some of the clearest evidence yet of how global warming can influence a regional weather pattern in often surprising ways.

With funds from the National Oceanic and Atmospheric Administration and Duke University, Li and her collaborators set out to examine seemingly confusing trends in the region’s rainfall in recent years. A severe drought in 2007 — the worst in Georgia in a century — famously led that state’s then-governor, Sonny Perdue (R), to hold a prayer service for rain. But two years later, Georgia faced unusual late-summer flooding.

To study the problem, Li and her colleagues closely analyzed summer precipitation patterns over a longer time period. The Southeast had highly reliable standardized data compiled from local rain gauges dating back to 1948, so they used that year as their starting point. Combing through the data, the researchers calculated a “standardized precipitation index” and then looked at how often the weather deviated significantly from the norm. As Li explains, “Our first task was to analyze the precipitation patterns over time. There was no obvious trend such as an overall increase in rainfall. But it soon became clear that there was increasing variability in the precipitation pattern in the region.”

The findings were dramatic: Li and her colleagues found that an abnormally wet or dry summer in the Southeast was more than twice as likely during the past 30 summers as it had been during the 30 summers before that. From 1948 to 1977, there were just two unusually wet and two unusually dry summers — technically, “rainfall anomalies that exceeded one standard deviation from the norm.” From 1978 to 2007, however, there were six unusually wet and five unusually dry summers. Using sophisticated statistical techniques to analyze the precipitation data, Li determined that both droughts and deluges had unquestionably increased over this time period in a statistically significant way. The question was: What could be causing these large swings in the region’s summer precipitation?

Li was well suited to study the question because she had been fascinated by fluctuations in the weather long before she ever heard of global warming. As a junior in high school in China, she accompanied her father on a field trip to a weather-forecasting observatory in Beijing. It made a strong impression on her. “My father was a chemical engineer, so I knew a little bit about working with chemicals in the lab. But when I saw the vast weather-forecasting computer and the technicians in their white coats, I was amazed. I think I knew from then on that I wanted to work in this field,” she says.  

When Li came to the United States to earn her doctorate in climate science, studying changes in the wet and dry seasons in the Amazon basin, she became fascinated by the effects of global warming on the mechanisms that drive these seasonal changes. As she puts it: “My work so far has focused on the interplay between global warming and natural variations in regional weather patterns.”

Li’s recent research on variability in precipitation in the Southeast is a case in point. Once she and her collaborators determined that swings of wet and dry summer weather had become more pronounced over the past six decades, they set out to see if they could correlate the precipitation patterns with other data on atmospheric and land-surface conditions. For example, they wondered whether the precipitation pattern was correlated with well-known climate cycles such as El Niño, which occurs across the tropical Pacific Ocean over a roughly two to seven-year period and affects weather patterns across North and South America. The group also looked for correlations with the so-called Atlantic multidecadal oscillation, a much longer-lived fluctuation in the surface temperature of the Atlantic Ocean that has an impact on drought in North America. But as Li explains, “The correlations were not statistically significant.”

“Based on our analysis,” Li says, “I am confident that the precipitation changes we are seeing are not caused by a natural variation like El Niño.” Instead, she explains, the group did find a strong correlation with data on the so-called Bermuda High, an area of high pressure that forms each summer in the ocean near Bermuda. It helps steer Atlantic hurricanes and plays a role in shaping weather as far away as northwestern Africa. 

Li and her colleagues found that the Bermuda High — officially known as the NASH (North Atlantic Subtropical High) — correlates closely with the precipitation fluctuations in their data. “The NASH was the only parameter we studied that passed the significance test.” And the more closely they looked at it, the clearer its role seemed to be.

To pinpoint the relationship, Li drew on data gathered from weather balloons, buoys, and satellites recording changes in the extent and location of the NASH over the 60-year period from 1948 to 2007. They found that in each decade during this period, the NASH had intensified, growing in area and migrating closer to the continental United States by a little over one longitudinal degree per decade. As the NASH intensified and moved westward, Li’s group found that its north-south movement correlated closely with the precipitation data they were studying. When the NASH moved north, it increased the likelihood of extremely dry weather in the Southeast that summer. When the NASH moved slightly south, it made extremely wet weather more likely in the region.

Unlike a natural variation like El Niño, Li explains, the changes in the intensity and western migration of the NASH result from global warming — primarily from steadily rising sea-surface temperatures in the Atlantic Ocean. As she notes, “Our analysis strongly suggests that the changes in the NASH are mainly due to anthropogenic warming.” In other words, human-induced climate change has caused a prevailing weather pattern to move closer to North America; when that high-pressure area wobbles slightly to the north or south, the consequences are felt more acutely in the Southeast’s regional rainfall compared to six decades ago.

Using statistical detective work, Li and her collaborators have charted a clear link between gradually warming temperatures and increasing swings from very wet to very dry summers in a region’s precipitation pattern. The research illustrates the power of drawing strong statistical correlations in understanding the relationship between the warming planet and variability in weather patterns. In this case, Li’s work shows, global warming seems to be increasing the chance of both droughts and floods in the southeastern United States in the years ahead.

“Our understanding of the mechanisms that drive the climate system is far from perfect,” Li says. But she adds that the statistical evidence presented in her research is powerful and should not be ignored. She likens it to the statistical correlations between smoking and lung cancer, which were clear long before the mechanisms of carcinogenesis were clearly understood. “In both cases, the statistical data can give you important information that can help to avoid risk,” she says.

Already climate scientists are drawing upon Li’s work to study similar relationships between high-pressure intensifications in the North Pacific and precipitation patterns in Asia. “Every research paper I have published raises more questions for me,” Li says. “We are making significant progress, but there are still many, many questions remaining. It makes it a very exciting time to be in the field.”

This is the 16th installment of America’s Climate Scientists: A series from the Union of Concerned Scientists. Click here to read all the climate scientist profiles.

The Union of Concerned Scientists is currently leading a campaign to elevate the voices of climate scientists and educate the public about the overwhelming scientific evidence for human-caused global warming. Learn how you can get involved at www.ucsusa.org/evidence.

When Bethany Bradley describes her research as probing the link between global warming and “alien invaders,” as she did in a recent journal article, the reader may be understandably disconcerted, especially since Bradley’s early graduate work involved mapping the surface of Mars. But rest assured: The alien invaders she studies today as a climate scientist and biogeographer at the University of Massachusetts, Amherst, are nonnative plant species such as kudzu (Pueraria lobata), endemic through the southern U.S., and purple loosestrife (Lythrum salicaria), which now clogs many of the nation’s waterways and canals.

While invasive plants are surely not as unsettling as the arrival of invaders from outer space, many such species have disrupted vast tracts of rangelands as well as farms and critical natural habitats. Bradley examines how global warming threatens to accelerate the spread of invasive plants, addressing a largely ignored risk and offering specific projections that can help ranchers, farmers, and conservation biologists anticipate what to expect.

As Bradley explains, “Invasive plant species are well suited to thriving in novel environments because of their ability to beat out competitors for resources. So it stands to reason that the more we disrupt the climate, the more these plants might be able to expand their reach.”

Bradley cautions, however, that invasive plant species are a diverse lot; accurately predicting how they will adapt to a warming climate means understanding a lot of complex interactions. Take, for instance, her recent work on cheatgrass (Bromus tectorum), a widespread scourge in North America’s Great Basin, bounded on the east by the Rocky Mountains and on the west by the Sierra Nevada range. Cheatgrass, Bradley says, was the species that drew her to study invasive plants in the first place. Early in her career as a graduate student in planetary geology at Brown University, Bradley recalls, one of her advisors gave her work on a project incorporating “remote sensing” satellite data of the earth. It seemed to her that she was seeing biology writ large. “I took to it immediately,” she says. “I had always been interested in biology but found it so much more fascinating on this kind of broad landscape scale.” Plus, compared with her studies of Mars, “it felt good to work on something closer to home that could be more directly useful to people.”

Cheatgrass was probably introduced into North America as seed contaminants in hay imported from Europe in the late 1800s. Farmers gave it that name because it pushes out wheat crops. In early spring, it looks like wheat but soon turns dry and tough, thereby “cheating” farmers of their expected yields. Worse yet, Bradley says, cheatgrass is 10 times more prone to fire than the sagebrush and grass habitat it most often displaces, and the resulting wildfires leave a burnt-out, barren landscape that facilitates the species’ further expansion.

Bradley says that cheatgrass, more than some kinds of vegetation, stands out particularly clearly in satellite data, “with a strong and distinctive green signal.” When she first observed it, she had no idea what it was, but the vast swath of land in the Great Basin that it covered was unmistakable. As she recalls, “Having grown up on the East Coast [in Connecticut], I was awed by the scale of the territory cheatgrass covered; I soon learned that it had come to dominate some 40,000 square kilometers (15,444 square miles) in this region — an expanse 10 times larger than Rhode Island, where I was in graduate school at the time.”

A decade ago, when she began graduate school, Bradley says, the field of climate change was not nearly as well established as it is today, and the effects of human-driven carbon emissions were less fully understood. Nonetheless, she was drawn to learning more about how urbanization and other human disruptions of the environment have helped foster the spread of invasive plant species. She adds that her current research into the effects of human-caused disruption of the climate is a natural extension of this earlier work.

“Climate change threatens to profoundly alter entire global ecosystems,” Bradley says. “Invasive species have been exciting to study as a window into such potential changes. Ideally, this kind of work can help us more realistically assess the risks and opportunities we face.” 

To this end, Bradley uses a technique called “bioclimatic envelope modeling,” which analyzes the climatic constraints within which a particular plant species thrives. Such an approach, she says, is well suited to studying how global warming might make a region increasingly vulnerable to invasions of nonnative species. In particular, Bradley says she tries to correlate the specific climatic factors most closely associated with a given species’ propagation. To accomplish this, Bradley first uses detailed satellite data and field observations to track the spread of an invasive species over time. Then, by comparing the species’ geographical spread to localized climate data, she teases out the climatic variables most closely associated with its successful spread. “One way of thinking about it,” Bradley says, “is that you test the changing map of the plant species’ whereabouts against every climate variable you can think of and see what factors float to the top.”

In the case of cheatgrass, Bradley determined that the variable most closely correlated with its propagation was spring rain or snow. “Precipitation later in the summer has a strong effect on cheatgrass’s competitors,” she says. “But precipitation in the spring, even more than temperature variables, correlates most closely with cheatgrass’s spread.” Presumably this spring water “gives cheatgrass its most potent advantage over competitors in quickly establishing itself.”

With that information in hand, Bradley consults each of the world’s 10 top climate models to see how much spring precipitation in a given geographical region it projects under different warming scenarios in the years to come. Then she can make detailed projections that link the projected climate conditions (resulting from our heat-trapping carbon emissions) to the geographical spread of this particular species.

Bradley has to contend with many challenges, including variations in the climate models’ projections and the confounding effects of normal year-to-year variability in weather patterns. Still, she is able to project with a good degree of confidence that cheatgrass is likely to move its range significantly northward invading new areas of Montana, Wyoming, Utah, and Colorado. Her research also indicates that significant portions of southern Nevada and southern Utah are likely to become climatically unsuitable for cheatgrass in coming decades, a finding that may allow land-use managers to help restore other, more desirable native species to these regions.

As Bradley puts it, “One of the great things about this work is that, unlike other areas of climate-change research where different constituencies have legitimate competing interests, in this case, all the interested parties are on the same side: just about everyone hates invasive species like cheatgrass and would like to contain their spread. It’s an aspect that makes this research feel more like I am working to understand and maybe even help solve a problem rather than doing battle.”

This is the 15th installment of America’s Climate Scientists: A series from the Union of Concerned Scientists. Click here to read all the climate scientist profiles.

The Union of Concerned Scientists is currently leading a campaign to elevate the voices of climate scientists and educate the
public about the overwhelming scientific evidence for human-caused global warming. Learn how you can get involved at www.ucsusa.org/evidence.

You’ve probably seen pictures of stranded polar bears and heard that global warming is causing the melting of Arctic sea ice — that is, floating ice formed from freezing ocean surface water. But you may imagine, as most people do, that this distant phenomenon is unfolding gradually over a centuries-long time frame.

Julienne Stroeve, a climate scientist at the National Snow and Ice Data Center in Boulder, Colo., has compiled detailed measurements that melt away any such misconceptions. Stroeve is closely monitoring the extent of Arctic sea ice, and her research shows that dramatic changes are occurring right now — far faster than most experts anticipated and with enormous consequences for the whole planet, not just the Arctic region.

For instance, during the warmest part of 2010, the total amount of Arctic sea ice — the so-called “seasonal minimum” — was the third-smallest ever recorded. The smallest and second-smallest seasonal minimums were measured in 2007 and 2008, respectively. Natural variability, including factors like cloud cover, can easily explain differences in melting from year to year, Stroeve notes. But the big news is that the smallest amounts of Arctic sea ice ever measured have all occurred in recent years. “Basically, ever since 2002, we’ve had one pronounced record minimum after another,” she says. “The data all point to a strong warming signal.”

Stroeve explains that highly reliable data on the extent of Arctic sea ice has been collected since 1978. From then until now, she has found clear evidence of a 30-year melting trend, which, she says, “cannot be easily explained away by natural variability.” But her work is even more notable for its findings about the speed of the change. Over this same 30 years, a relatively brief period, Stroeve has found that some 40 percent of the region’s summer (or more precisely, September) ice has melted.

The fast pace of melting is seen even more dramatically, she explains, when one considers the age of the Arctic ice. Many parts of the Arctic Ocean freeze each year during the coldest months. But only ice that lasts throughout the year gradually becomes thicker over the course of consecutive seasons. “In the 1980s, the Arctic contained roughly 386,000 square miles of ice that was determined to be at least five years old,” she says. Now, “at the end of the melt season in September, only 22,000 square miles of such older, thicker ice remains.” In other words, the region has already lost more than 97 percent of the thicker year-round ice that existed just three decades ago. As she explains, “all the climatic processes seem to be pushing rapidly toward a seasonally ice-free Arctic Ocean.”

Stroeve says that initially she was as surprised by the data as anyone else. “I didn’t think global warming was even happening in the early 1990s when I began this work,” she says. Back then, some climate models were projecting that carbon emissions would lead to a pronounced warming trend at the poles. But Stroeve was always more interested in actual measurements than in climate modeling. “I think I was lured into studying the poles by the prospect of adventurous fieldwork in Greenland or the Alps,” she says with a laugh.

The daughter of an aerospace engineer, Stroeve had always exhibited a strong aptitude for math and science and an adventuresome spirit. From childhood through her high school years, her dream was to be an astronaut, she says, and she might have continued on that track if she hadn’t realized that her susceptibility to motion sickness was a serious impediment to working in space.

Her love of adventure continues in her work today, in which she makes regular research trips to the Arctic and Greenland to measure ice thickness and other snow and ice measurements. It might not be space travel, but Stroeve says her fieldwork has been as exciting as she could have hoped. “When I first visited Greenland, it was the most stunning landscape I had ever seen,” she says.

Along with her research in the polar regions, much of the data Stroeve analyzes comes from satellites that detect passive microwave radiation. As she explains, the higher brightness of ice in the microwave part of the spectrum can be seen by satellites even through cloud cover. “In the often-cloudy polar regions, that makes it an incredibly useful tool, providing data in which we have a high degree of confidence,” she says. These detailed satellite measurements of Arctic sea ice led Stroeve to shed her initial doubts about global warming. “My views changed as I studied the emerging data,” she says. “With record low sea-ice extents year after year, it became clear that a significant warming trend was underway.”

Looking closely at the data, Stroeve realized that a phenomenon called Arctic amplification, a form of positive feedback, is accelerating the warming trend, causing it to occur many years sooner than most climate models had projected. Arctic amplification occurs primarily because water absorbs far more heat than ice does. On average, Stroeve explains, water absorbs almost 93 percent of all the incoming solar radiation, whereas the white surface of snow-covered ice reflects about 80 percent of incoming solar radiation back into space.

As more and more of the Arctic Ocean sea ice melts over the summer months, it hastens further warming, Stroeve explains. She and her colleagues at the National Snow and Ice Data Center have measured the effect, showing that in areas where summer ice has disappeared, local autumn air temperatures have been more than 5 degrees F higher than the long-term average.

The potential of such feedbacks to cause abrupt climate change as the Arctic Ocean becomes nearly ice-free in the warm season drew widespread attention to Stroeve’s work in 2007. In that record-breaking warm year, the Arctic Ocean lost more than one-quarter of its remaining ice. “Because new ice can’t get very thick in one season, it is more vulnerable to annual temperature changes, as we saw in 2007,” she says.

The possibility of sudden shifts in the region’s climate, and thus the planet’s climate, is the most frightening implication of her research, Stroeve says. The quick and volatile changes in Arctic sea ice remind us that the geological record contains clear evidence of abrupt climatic changes in the planet’s history. “We know that Arctic ice has historically helped keep the Northern Hemisphere cool,” Stroeve says. “Without it, given atmospheric circulation, the planet will certainly warm more quickly. But we don’t know enough about the system to fully project how swift the changes might turn out to be.”

The prospect of sudden climate change is certainly scary, Stroeve says. But she adds that, because the stakes are so high, her decision to study Arctic sea ice has proven a more exciting choice than she ever imagined. As she puts it, “Not a lot of people were looking at sea ice when I began my research. But especially after 2007, which took everyone by surprise, it has become something that climate scientists are intensely interested to know about.”

This is the 14th installment of America’s Climate Scientists: A series from the Union of Concerned Scientists. Click here to read all the climate scientist profiles.

The Union of Concerned Scientists is currently leading a campaign to elevate the voices of climate scientists and educate the public about the overwhelming scientific evidence for human-caused global warming. Learn how you can get involved at www.ucsusa.org/evidence.

The 2010 hurricane season is winding down, but teams of meteorologists are still working around the clock to track tropical storms as they continue to form in the Atlantic Ocean. Thomas Knutson tracks hurricanes too, but a little differently.

A meteorologist at the National Oceanic and Atmospheric Administration (NOAA), Knutson is trying to answer one of global warming’s thorniest and most consequential questions: What effect will rising global temperatures have on the frequency, structure, and intensity of future storms?

Five years after Hurricane Katrina devastated the Gulf Coast — causing more than 1,800 deaths and close to $100 billion in damage — there is little doubt about the urgency of Knutson’s research.

Knutson remembers the scene when he first visited Gulfport, Miss., a few years ago; 30-foot-high water marks testified to Katrina’s devastating surge. “Storms like Katrina are thankfully rare,” he says. “But when you see how hard they hit, it reminds you how bad things can be and how vital it is to prepare and plan ahead.”

The extreme danger posed by hurricanes like Katrina motivates Knutson and his team at NOAA’s Geophysical Fluid Dynamics Laboratory, which is located on the campus of New Jersey’s Princeton University.

Growing up in a tiny town in rural Virginia, the teenage Knutson first encountered climate change when he read an article about the phenomenon in the Washington Post. “It captured my imagination,” he recalls. Knutson was influenced too by his father’s work as a geologist at a local zinc mine. “I was steeped,” says Knutson, “in a certain ‘geological’ way of looking at the world.”

Knutson earned an undergraduate degree in computer science, then went on to graduate studies in meteorology at the University of Wisconsin-Madison. For the past two decades, he has been a research scientist at NOAA. But Knutson is quick to acknowledge that teasing apart the relationship between global warming and hurricanes is an exceedingly complex business.

First, says Knutson, there is the data problem. “Reliable satellite data for hurricane intensities exists only as far back as about 1980,” he says. Before that, scientists like Knutson and his colleagues must rely on what he calls “a hodgepodge” of information.

“We know a lot about hurricanes that made landfall for the past century or more,” he explains. “But if a hurricane didn’t hit land, it’s a different story, and, of course, many don’t hit land.” Before the mid-1940s, the only information about those tropical storms and hurricanes that never struck land came from ships that happened to be in the vicinity and recorded wind strengths in their logs. In recent decades there have been just 11 named tropical storms each year in the Atlantic. On average, only a few (2.3 on average) have developed into hurricanes of category three or higher. The relative rarity of these large hurricanes — what Knutson calls “the small size of the signal” — makes statistical analysis even trickier. 

Today, the evidence is overwhelming that carbon dioxide and other heat-trapping gases are warming summer sea-surface temperatures to levels that allow hurricanes to develop. But “to predict how hurricane activity may change,” says Knutson, “we need to know a lot of fine-grained detail about things like the local pattern of sea-surface temperature, not just the global average.” Current climate change models use grids that are 62 to 124 miles in size, roughly the distance between New York City and Philadelphia. That scale, says Knutson, is “roughly two orders of magnitude too crude for the purpose of simulating hurricane intensities.”

To overcome the limitations of data and scale, Knutson and his team have developed an ingenious strategy. “Rather than wait a few decades until climate models offer enough resolution to predict storm formation,” says Knutson, his team “telescopes in.” They borrow data about future climatic conditions from 18 separate world climate models and plug it into the more detailed regional weather forecasting models that NOAA uses to track current storms.

Using supercomputers, Knutson’s team tested the approach by entering atmospheric and sea-surface temperatures measured in recent years, including the 2005 hurricane season that produced Hurricane Katrina. Only after determining that their detailed regional model could offer a realistic picture of intense hurricane activity in those years did they run the model with anticipated future conditions.

Knutson’s results so far show both good news and bad. On the positive side, global warming is likely to mean less frequent hurricanes in the Atlantic, largely because higher wind speeds in the upper atmosphere will disrupt many of them. But models indicate that storms which do materialize will likely grow to be more intense. Knutson’s latest modeling study projects that the number of category four and category five hurricanes in the Atlantic will double by the end of this century, making a catastrophic storm like Katrina considerably more likely.

Knutson also warns that today’s climate models uniformly project that the hotter atmosphere brought about by global warming will retain more moisture. For developing hurricanes, this means more intense precipitation — some 20 percent higher rainfall rates, according to Knutson’s calculations. In addition, he says, current estimates of rising sea levels over the coming century mean that the storm surges caused by intense hurricanes are likely to be more devastating than ever before.

Knutson and his team are already refining their hurricane research. Ultimately, global climate models should become fine-grained enough to predict the formation of storms. Meanwhile, coastal communities are paying close attention to Knutson’s findings so they can brace for these rare but intense hurricanes and make better plans to cope with them.

“We live in a unique time,” Knutson says. “Unfortunately, we have been running a global experiment that is perturbing the climate system in profound ways, and climate affects many things in our natural world. I’m glad to work on a project that can at least help us predict and anticipate some of these effects so we can make more informed decisions.”

This is the 13th installment of America’s Climate Scientists: A series from the Union of Concerned Scientists. Click here to read all the climate scientist profiles.

The Union of Concerned Scientists is currently leading a campaign to elevate the voices of climate scientists and educate the public about the overwhelming scientific evidence for human-caused global warming. Learn how you can get involved at www.ucsusa.org/evidence.

The ocean has been our savior.

Besides generating about two thirds of the oxygen we breathe, oceangoing phytoplankton — those floating microscopic plants that form the base of the aquatic food chain — absorb about a third of all the carbon dioxide we pump into the atmosphere. In this way, the oceans have managed to slow the buildup of heat-trapping greenhouse gases and stave off even more dramatic warming of the planet.

But John Guinotte and colleagues are discovering that the critical role of “carbon sink” comes at a potentially devastating cost for the world’s oceans: acidification.

Guinotte is a coral specialist at the Marine Conservation Biology Institute in Bellevue, Wash. The changes he sees in ocean chemistry spell trouble for the coral that he studies closely. If the acidification process continues on its current trajectory, it poses a dire threat to the whole marine ecosystem.

“What I’m really concerned about with ocean acidification is that we are facing the prospect of a crash in marine food webs.” says Guinotte. “There is no question that many of my colleagues in marine science are scared about what is happening. We know we need a more precise understanding of the changes and biological responses now under way — and we need it as quickly as possible, before it is too late to turn things around.”

Guinotte has dedicated his life to the study of coral, especially the less well understood deep-sea varieties. Growing up in rural Kansas, his only exposure to corals was through the pages of National Geographic. But that changed when he learned to scuba dive at his grandfather’s winter home in the Florida Keys. The experience, plus his interest in biology and geography, led him to Australia, where he earned his Ph.D.

Guinotte still remembers the thrill of exploring Australia’s Great Barrier Reef for the first time. “I was absolutely blown away by the abundance and diversity of coral,” he recalls. At that time, back in the late-1990s, scientists were increasingly concerned about coral bleaching caused by environmental stresses such as warming ocean temperatures. Those threats remain, Guinotte says, but ocean acidification may be an even more serious and intractable problem.

On the macro scale, Guinotte explains, the chemistry of ocean acidification is relatively clear. Based on some 25 years’ worth of measurements scientists know that oceans absorb about 22 million tons of carbon dioxide every day. The oceans are vast. But even so, the absorption of CO2 is now occurring at such an unprecedented rate that ocean chemistry is approaching a state not seen in many millions of years. Guinotte fears that many marine species might be unable to adapt quickly enough to survive these dramatic changes.

As carbon dioxide is absorbed by seawater, hydrogen ions are released. This lower the pH, making the water more acidic. Measurements indicate that Earth’s oceans are already about 30 percent more acidic than they were before the industrial revolution. As the number of hydrogen ions has risen, the number of carbonate ions available in seawater has gone down. This carbonate deficit makes life more difficult for the “marine calcifiers,” species such as coral and shellfish that use carbonate to build their skeletons and protective shells.

“Ocean water becomes increasingly corrosive to calcium carbonate,” says Guinotte. “A reduction in carbonate ions not only impedes corals’ ability to build their skeletons, but once the calcium carbonate drops below critical levels, the ocean erodes the framework they have built up previously — the reefs upon which corals live.” Even if select coral species can survive ocean acidification, Guinotte says, when the coral reefs begin to dissolve, the effects on the entire marine ecosystem are likely to be devastating.

Scientists know from the fossil record that reefs which sustained damage from high atmospheric concentrations of CO2 in the geologic past took millions of years to recover. “Given that we need to think in human time scales, it means we’re playing for keeps here,” says Guinotte. “To me, it sometimes seems like a school bus full of children heading for a cliff. Somehow we have to slow it down enough to find some real solutions.”

Because of the very clear potential for ocean acidification to effect everything from the tiniest oxygen-providing phytoplankton to the larger fish that feed in the coral reefs — or, as Guinotte has written, “from the shallowest waters to the darkest depths of the deep sea” — the threat to humankind is immense.

To figure out precisely how much acidification many varieties of coral can tolerate, and what we can do to preserve the health of the marine ecosystem, Guinotte argues for a coordinated research effort that tackles every aspect of the problem. That includes better monitoring of ocean carbon; closer tracking of calcifying organisms and more laboratory and field studies of their physiological responses to increasingly acidicity; and more detailed studies that model the threat to the marine ecosystem as a whole. Some of this work is under way, but too much of it has been conducted in piecemeal fashion. Only a more intensive, coordinated effort, says Guinotte, can provide the detail necessary for policymakers to develop strategies that protect critical species, habitats, and ecosystems.

“From the standpoint of the oceans,” Guinotte says, “there is no escaping the fact that we are going to need major reductions in our CO₂ emissions — something like 80 to 90 percent. When we see governments arguing about reductions of 10 to 15 percent, I think all of us in the marine science community need to say that CO₂ reductions of this scale are simply not going to be sufficient. We have to get off fossil fuels.”

The fossil record shows that high CO₂ concentrations have likely played a big role in mass extinctions of marine life in the past. “If marine systems start to crash, it may well be too late to stop the train,” says Guinotte. “Governments are likely to panic and make irrational decisions; international tensions could certainly heat up. These are the kinds of things that keep me awake at night. I continue to hope we can get it turned around. But it will take political will, and so far, that has been in short supply.”

This is the 12th installment of America’s Climate Scientists: A series from the Union of Concerned Scientists. Click here to read all the climate scientist profiles.

The Union of Concerned Scientists is currently leading a campaign to elevate the voices of climate scientists and educate the public about the overwhelming scientific evidence for human-caused global warming. Learn how you can get involved at www.ucsusa.org/evidence.

If you’re one of the tens of millions of people who live in the southwestern United States, get ready for drier weather. That’s the message from Richard Seager, a climate scientist at Columbia University’s Lamont-Doherty Earth Observatory. The American Southwest, says Seager, is soon likely to experience a “permanent drought” condition on par with the Dust Bowl of the 1930s.

That rather frightening prediction is the most likely scenario for the region, given the global warming now underway. “It is a matter of simple thermodynamics,” says Seager. “The region will face a considerable increase in aridity over the coming decade.”

The Southwest is as dry as it is because the local atmospheric flow tends to export far more moisture than storms can carry into the region. This is the case in other parts of the so-called subtropics, those areas directly north and south of the equatorial tropics. But as earth’s atmosphere becomes laden with heat-trapping greenhouse gases, it will be able to retain even more moisture. That means more evaporation from lakes and rivers, more moisture loss from plants, and drier soil.

A critical player in this drying cycle is the planetary-scale circulation system known as the “Hadley cell.” This vast atmospheric system links rising air near the Equator with descending air in the subtropics, giving rise to the subtropical jet streams.

In the northern hemisphere the jet stream flows west to east across North America. Rising moist air condenses and forms thunderstorms in the tropics, but the moisture is largely lost by the time the air descends at subtropical latitudes. That’s why most of the world’s deserts are situated in the subtropics.

The Hadley cell is growing. Its expansion above a larger swath of the American Southwest, along with a shifting of the jet stream and many storms northward, is a worrisome trend, says Seager. It means there is little chance that the Southwest can avoid becoming drier in the coming decades. In fact, when Seager’s team analyzed some 49 computer projections of the region’s likely future climate, using 19 major climate models, all but three scenarios agreed: drought ahead.

Seager has been tracking changes in precipitation for nearly a quarter century. For his graduate thesis at Columbia University he used computer models to try and understand the role of sea-surface temperatures in driving precipitation patterns in the tropics and around the world.

Growing up in a working-class family in Norwich, England, Seager spent a lot of time outdoors with his family. Cycling and hiking the hills of England and Scotland sparked his interest in the physical world. It didn’t hurt his current professional interests that he grew up with “more weather than you would wish on anyone.”

One of his undergraduate tutors was the climate scientist Ann Henderson-Sellers. She had recently returned to England after working in James Hansen’s renowned climate science research center in New York. “She’s the one who suggested that I apply to graduate school in climate science in the U.S.,” says Seager. He and his brother were the first members of their family to earn college degrees.

Moving to New York for a graduate degree was a big step — and, as it turned out, a lasting one. Except for a stint as a postdoc at the University of Washington in Seattle, Seager has remained at Columbia to conduct climate research ever since.

Seager recognizes that the stakes of his drought research are high. “The prospect of a drought on par with the 1930s is a matter of serious concern,” he says. “With some two million people displaced, the Dust Bowl was probably the worst environmental disaster in the nation’s history — even counting the current oil spill in the Gulf.”

Seager is quick to add, however, that many features of the Dust Bowl are unlikely to be repeated. For one thing, he says, “we have learned an awful lot about soil conservation since the 1930s.”

As severe as the impending drought conditions now appear, Seager also emphasizes the vital mitigating role played by the Colorado River, which carries an enormous volume of water to the southwestern United States. The region, he notes, now diverts between 80 and 90 percent of that water for agricultural uses, but that could be changed.

Other regions are not as lucky. The Colorado River is barely a trickle of water by the time it reaches the border with northern Mexico, where the potential for water shortages is more immediate and more likely to lead to the displacement of people in coming years. 

Seager believes that part of his job is to inform people about these climatic changes. He often briefs water managers throughout the region, including those with the California Department of Water Resources and at the U.S. Bureau of Reclamation, which manages the Colorado River.

“It’s a comparatively small part of my job, because mostly I’m focused on doing the science,” Seager says, “but it is an important part. Much of our funding comes from the National Oceanic and Atmospheric Administration (NOAA), and a reasonable condition of the grants is that we do something to make sure the information gets out.”

One common complaint is that his work is neither precise enough nor at a large enough scale to be useful to water managers. He feels their pain. “There is still so much natural variability in the weather that I cannot say with certainty what will happen in their particular neck of the woods and exactly when,” he says. But he is hard at work trying to improve his modeling of the region’s climate.

For the past several years Seager has been studying naturally occurring droughts in the American Southwest all the way back to the Middle Ages. The man-made forces that are driving today’s climate change are clear, he says. But the extent to which naturally occurring cycles might mitigate or exacerbate the impending drought remain uncertain.

“When I first went into this field, it had little perceived practical relevance,” says Seager. “The field has developed rapidly to a point where it can offer practical predictions, and the goal now is to make these as precise as we possibly can.”

Water managers in the Southwest seem to be paying attention, and even taking action. “They understand that it’s going to get drier,” says Seager. “So it is probably not a good idea for them to sit around and wait until our models get better.”

This is the 11th installment of America’s Climate Scientists: A series from the Union of Concerned Scientists. Click here to read all the climate scientist profiles.

The Union of Concerned Scientists is currently leading a campaign to elevate the voices of climate scientists and educate the public about the overwhelming scientific evidence for human-caused global warming. Learn how you can get involved at www.ucsusa.org/evidence.

Inez Fung is on a mission to find and account for every gram of heat-trapping carbon dioxide on the planet. And she knows where most of it is hiding.

Fung is the director of the Berkeley Institute of the Environment at the University of California-Berkeley. Her work has led to a more complete understanding of the current and future role played by Earth’s so-called “carbon sinks” — features such as oceans and forests that suck carbon dioxide out of the air. Fung’s research shows that when the role of these carbon-absorbing mechanisms is taken fully into account, global warming is likely to accelerate even faster than scientists previously believed.

Why study carbon sinks? Because the planet’s ability to absorb carbon dioxide is a vital and tricky part of the climate-change equation. Up until now, Earth’s land, vegetation, and oceans have soaked up roughly half of all the heat-trapping CO2 we have emitted by burning fossil fuels. Fung’s research analyzes whether our carbon sinks can keep pace with today’s unprecedented levels of CO2 emissions. The stakes are high, because any reduction in the Earth’s ability to absorb CO2 could dramatically increase the swiftness and severity of global-warming processes now underway.

As high as the stakes may be, the planet’s so-called “carbon dynamics” are tough to master, requiring a detailed knowledge of everything from atmospheric transport models to the mechanics of photosynthesis. Luckily, Fung is well suited to the task.

She has always loved math. “It’s no doubt irksome to some people,” she says, “but I see everything through the lens of mathematics.” She graduated from MIT with a degree in applied mathematics. After deciding to stay on for a PhD, she took the field of meteorology by, well, storm, when she used math and fluid dynamics to explain the spiral shape of hurricane rain bands.

Weather is Fung’s other passion. As a child growing up in Hong Kong, she monitored the harbor lights, which were put in place to warn of an impending typhoon. “Of course, back then,” she says, “we were mostly excited about [typhoons] because it meant school would be canceled.”

After grad school, Fung joined a climate-modeling team led by the well-known climate scientist James Hansen at NASA’s Goddard Institute for Space Studies at Columbia University in New York. It was there, she says, that she started thinking in earnest about Earth’s carbon cycle. It seemed obvious to her that carbon sinks had to be included in any analysis of climate change. Without that factor, she says, the results would be as incomplete as “trying to make a budget by looking at your income without considering your expenses.”

Fung learned about carbon sinks on her own, from scratch. She married Jim Bishop, a marine chemist, and she and her husband often went on camping trips with other scientists. On hikes and around the campfire, she would buttonhole colleagues to learn everything she could about the arcane aspects of the carbon cycle, often taking notes on what they told her. Before long, she determined that Earth’s uptake of carbon “could be reduced to seven equations with seven unknowns.”

Fung’s analysis derives from the fact that there are a finite number of major types of carbon sinks. The oceans are one type. Researchers know that oceans absorb heat-trapping CO2 from the atmosphere as surface water mixes with air. Marine creatures incorporate this dissolved CO2 into their shells.

Terrestrial plants also trap carbon. They take in CO2 and, through the process of photosynthesis, turn it into carbohydrates, which are stored in leaves, trunks, or roots. When leaves fall or the plants die, soil microbes decompose the plant detritus and return the CO2 to the atmosphere.

Before Fung’s work, most scientists studying the carbon cycle had focused on one or another specific carbon-trapping mechanism, such as the ecophysiology of leaves from a particular tree. As valuable as that research is, it is too fine-grained for Fung’s purposes. As she puts it, “[It's] like trying to determine how the economy is doing by looking only at one corner store.”

Fung prefers to think about the carbon cycle at the biosphere scale. The rise in overall levels of CO2 grabs all the headlines, but Fung and her team focused on the seasonal fluctuations in CO2 data collected from around the world. They noticed that concentrations reach their highest levels in May, before the growing season begins and photosynthesizing new foliage draws the levels down. “We look at these records in great detail to derive everything we can about the biosphere,” she explains. “It is like you can see the Earth breathing.”

Before long, Fung’s detailed data analysis helped her build a large-scale computer model to represent the geographic and temporal variations of CO2 sources and sinks. More recently, Fung has coupled her carbon-cycle model with existing large-scale computer climate models to project how land and ocean carbon sinks are likely to change as global warming proceeds. 

One major finding: droughts have already diminished the carbon absorbing capacity of the land and will continue to do so. Previous greenhouse experiments suggested that elevated CO2 levels caused plants to grow bigger and faster, an effect known as CO2 fertilization. The implication was that land-based carbon sinks — that is, plants — might be able to keep pace with higher CO2 levels. Fung’s modeling shows that on a global scale, regional droughts are likely to curtail this effect.

Her model also projects that the tropics are likely to become hotter and drier in summer months. As that happens, plants will absorb less carbon dioxide as a way to avoid water loss. In fact, atmospheric measurements over the past decade have already confirmed this effect. Fung says that her research shows that soil moisture is a key variable, and she worries that increasing regional droughts will further hasten warming trends.

After initial skepticism, Fung’s colleagues now pay close attention to her analysis. Her work is widely cited, and she has won many accolades, from attaining membership in the National Academy of Sciences to being named one of Scientific American‘s 50 most influential scientists in 2005. Her life and work are the subject of an online comic strip aimed at middle schoolers.

Fung’s efforts will get a big boost with the 2013 launch the new Orbiting Carbon Observatory (OCO-2) satellite. OCO-2 will collect an unprecedented volume of data on the levels of heat-trapping CO2 in the atmosphere. As Fung explains, she will go from being able to draw upon roughly one hundred observations of near-surface CO2 concentrations every two weeks — from remote locations around the world — to a million. In addition, OCO will record CO2 levels through the entire atmospheric column, eliminating the need to guess about variations at different altitudes.

Not surprisingly, Fung sees the tough policy choices that global warming presents as a complex math problem. “In considering something like climate change, the political arena has to weigh many, many variables, from economic to environmental considerations. In math we call this a weighting function, and it all depends on how you weigh these different variables. I don’t know the best way to do that. What I do know is that my role is to offer the most accurate analysis I can of what is happening.”

“Unfortunately,” Fung says, “I don’t think we scientists have done very well communicating the issues to the public. We do a lot of talking to one another. But I still haven’t seen any of my friends on Oprah yet. I’m afraid we are not broadcasting our findings on the right wavelength.”

This is the 10th installment of America’s Climate Scientists: A series from the Union of Concerned
Scientists. Click here to read all the climate scientist profiles.

The Union of Concerned Scientists is currently leading a campaign to elevate the voices of climate scientists and educate the public about the overwhelming scientific evidence for human-caused global warming. Learn how you can get involved at www.ucsusa.org/evidence.

So, how do we know that human byproducts — namely emissions from our tailpipes and smokestacks — are responsible for warming the planet? To Benjamin Santer, a climate scientist at the Lawrence Livermore National Laboratory in California, the answer is in the evidence.

Just as criminals leave fingerprints and DNA at the scene of a crime, climate change culprits leave distinct signatures or patterns that scientists can find — if they look closely enough. And hardly anyone has been looking longer or more carefully than Santer.

More than 23 years ago, just out of graduate school, Santer joined the Max Planck Institute for Meteorology in Germany. Through his work there with geophysicist Klaus Hasselmann, Santer became one of the first scientists in the world to begin to separate the signs of human influence on global climate from what he calls “the background noise of climate variability.”

Their research took advantage of early computer climate models which were generating a wealth of information about these human “fingerprints.” Scientists had documented for years that the average surface temperature of the planet was rising. But Santer and Hasselman began looking more closely at the geographical and altitudinal patterns of that warming.

The factors that might account for global warming — what climate scientists call “forcings” — operate in different ways. If Earth’s warming was caused by an increase in the sun’s energy output, explains Santer, “you would expect to see warming from the top of the atmospheric column straight down to the surface.” Warming caused by, say, massive volcanic eruptions would present a distinctly different profile.

The dust from volcanic eruptions can reach the upper portions of Earth’s atmosphere, and linger there for several years. Because volcanic dust absorbs sunlight, preventing its warmth from penetrating to the Earth’s surface, climate data would show cooling in the troposphere (the atmospheric layer closest to the Earth’s surface) and heating in the stratosphere (the layer above the troposphere).

But this pattern, says Santer, is “not at all what the data show.” His research, now replicated by many others, documents a telltale warming of the troposphere and a cooling of the stratosphere. This is the precise “greenhouse effect” fingerprint that scientists, since the 1960s, had predicted as increasing amounts of heat-trapping carbon dioxide from fossil-fuel emissions built up in the atmosphere.

Because of his groundbreaking work, Santer was selected as the lead author on a chapter of the 1995 report issued by the Intergovernmental Panel on Climate Change (IPCC). That year, for the first time, the report said that “the balance of evidence suggests a discernible human influence on global climate.” That measured statement has, of course, been dramatically strengthened in the latest IPCC report, which puts the likelihood that human activities have been the main cause of warming since the middle of the twentieth century at greater than 90 percent.

Santer’s research led to widespread acclaim from his colleagues and earned him many accolades, including a MacArthur Genius Grant. But his high-profile role in the 1995 IPCC report also made him a target. After the report was released, the Global Climate Coalition, an industry group funded mostly by oil companies, claimed Santer had altered the IPCC’s findings. He had not.

“Nothing in my university training prepared me for what I faced in the aftermath of that report,” Santer says. “You are prepared as a scientist to defend your research. But I was not prepared to defend my personal integrity. I never imagined I’d have to do that.”

Fifteen years later, the evidence that human activity is causing global warming is stronger than ever, and accepted by the overwhelming majority of scientists. Our understanding of climate fingerprinting has also become far more sophisticated and now shows human impact on changes in ocean temperatures, Arctic sea ice, precipitation, atmospheric moisture, and many other aspects of climate change.

Some of Santer’s more recent work, for instance, addresses changes in the height of the tropopause — the boundary between the troposphere, the more turbulent lower layer of Earth’s atmosphere, and the more stable stratosphere above. (Between 5 and 10 miles above the Earth’s surface, evidence of the tropopause can be seen in the flat, anvil-like top of a thundercloud.) Measurements over the course of several recent decades have shown that the tropopause has risen markedly. By studying tropopause changes in computer climate models, then comparing model output with actual observations, Santer was able to show that both the warming of the lower atmosphere and cooling of the stratosphere led to a rise in the height of the tropopause — and that the observed rise matched the fingerprint of an increase in heat-trapping gases. “Nobody had looked at it before,” Santer says. “But the data showed clearly that natural causes alone simply could not provide a convincing explanation for the observed change.”

All the climate fingerprinting research to date, has arrived at the same conclusion, says Santer; namely, that “natural causes cannot provide a convincing explanation for the particular patterns of climate change we see.” That, he says, is why scientists “have come to have such confidence in our understanding of what is happening — not because of the claims of any one individual, but because of the breadth of scientific work and reproducibility of the results.”

Despite the confidence of the scientific community, Santer is still regularly harassed by climate deniers. Soft-spoken, meticulous, and cautious by nature, he is an unlikely lightning rod, yet he often receives hate mail — and worse. Late one evening several years ago, he answered his doorbell to find a dead rat on the doorstep and a driver shouting curses from a Hummer that was zooming away down the street. Santer vented some frustration in private emails to former colleagues in the Climate Research Unit at the University of East Anglia in Britain. His emails, along with those from other scientists, were among the cache stolen and publicized in an effort to distract and mislead the public just before the 2009 climate summit in Copenhagen.  

Santer knows a thing or two about tough spots. When he was 11-years-old, his father’s work for an international food service company took the family from suburban Maryland to Germany. Santer didn’t speak a word of German. His family enrolled him in a British army school, where he was the only kid who didn’t live on the military base or play soccer, and he had never even heard of cricket. He still remembers his teacher whacking him on the head with a rolled-up lesson book for writing in pencil instead of using the pen and inkwell on his desk as the European kids had been taught to do. “Let’s put it this way,” he says; “it was character-building.”

Today, Santer perseveres with much the same attitude. Unbowed by what he politely calls the “forces of unreason,” he continues to turn out world-class research and make himself available to speak about the scientific data on global warming. “I continue to believe that if the science is credible, people will do something about it,” he says.  ”Since my research is funded by the federal government, I’ve learned that my job is not just to do the best scientific research I can. I also have a responsibility to try to explain to people what it means, so an informed electorate can make good choices.”

This is the ninth installment of America’s Climate Scientists: A series from the Union of Concerned Scientists. Click here to read all the cl
imate scientist profiles.

The Union of Concerned Scientists is currently leading a campaign to elevate the voices of climate scientists and educate the public about the overwhelming scientific evidence for human-caused global warming. Learn how you can get involved at www.ucsusa.org/evidence.

Exactly how much did the sea level rise three million years ago? Okay. Probably not a question you’ve asked yourself lately. But the question and, more importantly, its answer are significant. They will help scientists understand how fast and how high our current sea levels are likely to rise as today’s global warming trend melts the remaining ice sheets in Greenland and Antarctica.

Fortunately, there are researchers wrestling with the problem. Chief among them is Maureen Raymo, a paleoclimatologist at Boston University. Raymo heads a multidisciplinary team that is spending its second summer digging for evidence in the desert of Western Australia.

Raymo is drawn to really big questions, a really good thing considering her line of work. In order to understand the connection between Australia’s prehistoric sea level and today’s climate conundrum, you have to be able to think on a grand geologic and planetary scale.

It turns out that global temperatures fluctuated long before humans began adding heat-trapping gases to the atmosphere. The fluctuations were the result of subtle, recurring shifts in the Earth’s orbit as it travels around the sun.

The tilt of the planet varies predictably on a 41,000-year cycle. More tilt means warmer summers and colder winters at high latitudes; less tilt means cooler summers and milder winters. Scientists believe this 41,000-year variation (known as the Milankovitch cycle), along with other variables — like the shape of Earth’s orbit and the seasonal timing of when Earth is closest to the sun — have influenced the advance and retreat of glaciers and thus the onset of ice ages as more or less solar radiation reaches Earth at mid-to-high latitudes.

Such observations from the complex field of orbital dynamics have helped scientists like Raymo identify the cyclic changes in the Earth’s climate. But these “orbital forcings” are only a piece of the climate puzzle. The Earth’s geologic history also plays a role. 

Three million years ago, during what is known as the mid-Pliocene climate optimum, Earth was a much warmer place. In fact, this period marked the most recent time during which the climate was consistently warmer than it is today for an extended period. During the mid-Pliocene climate optimum, global temperatures were as much as 5.4 degrees F above today’s averages. As a graduate student, Raymo proposed that this warmth was due to higher levels of carbon dioxide in the atmosphere at the time, a hypothesis that is still being tested today.

Scientists are not sure how high the sea level was during the mid-Pliocene climate optimum. Some past research suggests that it was just 15 feet higher than it is today; other research puts it as much as 100 feet higher. Raymo hopes to significantly narrow this gap. “If we can determine exactly where the shoreline was three million years ago,” she says, “we can tell a lot about how much ice remained in Greenland and Antarctica during this warm period.”

To map the ancient shoreline, Raymo and her team work their way inland from the Australian coast in search of fossilized coral reefs and other evidence that the land was once covered by ocean. “Each time you add a data point,” Raymo says, “you help to calibrate existing climate models in an important way and build a broader knowledge base for future experiments.”

Raymo is quick to add that uncertainty about a specific question such as the maximum sea level in the Pliocene Epoch should not be equated with uncertainty about today’s overall global warming trend. Climate-change skeptics who jump on uncertainties to dispute global warming, “completely miss the point,” says Raymo. “Science is always evolving intellectually, processing and incorporating the latest information. Scientists always want more knowledge. They always say ‘I want to try to test out that new idea.’”              

“Even so,” Raymo says, “everything I’ve learned about the dynamism of the planet’s climate places me, like most all of my colleagues, strongly in the camp that says we need to take preventive action to keep the planet from warming further. I am extremely concerned that we’ve made such large changes in the composition of our atmosphere with so little understanding of the consequences. The path we are on is completely anomalous to anything that has gone before.”

Paleoclimatologists like Raymo focus on vast stretches of geological time, studying variations in the earth’s climate over the course of the planet’s history, gleaning evidence from a variety of sources to track which parts of the planet were once covered by glaciers or oceans. They see geologic evidence of glaciers in shifts of the terrain and in the rocks and other material the glaciers left behind; they find chemical evidence of oceans in variations in the ratios of isotopes in sedimentary rocks.

The tricky part, of course, is to figure out how to use the hard evidence available today to draw conclusions about past climate. It is a part of her job that Raymo particularly enjoys. “I love thinking ‘How does the Earth work?’” she says. “I have always felt a natural ability to think about the planet, almost as though I were looking down on it from above.”

This kind of big picture thinking runs in Raymo’s family. Her father, Chet Raymo, is a science writer well known for his musings on grand-scale topics in the natural world. But when asked how she got interested in science, Raymo credits Jacques Cousteau. At age seven, she was captivated by one of the French undersea explorer’s television programs. “From then on I decided I wanted to be an oceanographer,” she says. That passion stayed with her all the way to college, when she was seduced anew by the cutting-edge climate research of geologists John Imbrie and William Ruddiman, who became her mentors.

In her research, Raymo tackles many aspects of paleoclimate and orbital forcings. In 2006, she hypothesized, in the journal Science, about why the 41,000 year Milankovitch cycle appears less pronounced over the past 500,000 years. “It was a question that I had literally been thinking about since graduate school some two decades earlier,” she says. No one knows the answer yet. But Raymo’s theory — that ice growth at one pole occurs simultaneously with ice decay at the other, thereby canceling out the signal of ice volume change in global climate records — has sparked a great deal of new research and debate. If she’s right, then earth’s climate is even more dynamic and complex, especially in the area of the Antarctic ice sheet.

While orbital forcings offer a fascinating window into the historical mechanisms of climate variation, Raymo emphasizes that they are overshadowed today by human-driven effects on the climate. “People sometimes ask me: ‘When will the next ice age be?’” she says. “The answer is that I’m pretty sure we have already prevented it. Like it or not, we are now the main drivers of the climate, even though so far we’ve been doing it completely by accident.”

This is the eighth installment of America’s Climate Scientists: A series from the Union of Concerned Scientists. Click here to read all the climate scientist profiles.

The Union of Concerned Scientists is currently leading a campaign to elevate the voices of climate scientists and educate the public about the overwhelming scientific evidence for human-caused global warming. Learn how you can get involved at www.ucsusa.org/evidence.